a) Field of the Invention
The present invention relates to an optical system for non-flexible endoscopes and more specifically a relay lens system for non-flexible endoscopes.
b) Description of the Prior Art
A non-flexible endoscope ordinarily has a composition such as that shown in FIG. 1, consists of a non-flexible insert section 1 which is elongated for insertion into cavities of living bodies or the like, and a grip section 2 which is located outside the cavities and held by an operator's hand or a non-flexible endoscope holder. An observation optical system O which allows observation of an object located in a cavity is disposed so as to extend from the insert section 1 to the grip section 2. A portion of the observation optical system O which is disposed in the insert section 1 consists of an objective lens system disposed on a side of a distal end and serves for forming a real image of an object, and a relay lens system R for relaying the image formed by the objective lens system to a portion of the optical system disposed in the grip section. The objective lens system and the relay lens system are disposed coaxially in an optical system holding tube 3 which is accommodated in the insert section 1. A portion of the optical system disposed in the grip section comprises an eyepiece system E which allows observation of an image of the object which is relayed by the relay lens system. Further, a video camera for non-flexible endoscopes is disposed on an eyepiece mount for allowing observation of the image of the object on a TV monitor since video observation is indispensable for surgical operations with a non-flexible endoscope.
A relay lens system which is to be used with such a non-flexible endoscope or the like is configured to relay an image a plurality of times. A module used for relaying an image for a single cycle distance through the relay lens system will hereinafter be referred to as a relay lens unit R1, R2 or R3.
A lens system disclosed by Japanese Patent Kokoku Publication No. Sho 49-5993 is known as a conventional example of such a relay lens system for non-flexible endoscopes. A conventional relay lens system, such as the conventional example mentioned above, is usually configured to relay an image a plurality of times using a plurality of relay lens units which have the same composition. For minimizing the amounts of aberrations that are produced by the relay lens system as a whole, it is necessary to correct aberrations independently in each of the relay lens units. For this reason, each relay lens unit mostly comprises a negative lens element. In such a case, however, there is posed a problem that the relay lens unit requires a large number of lens elements and has a high manufacturing cost for the relay lens system.
FIGS. 2A, 2B and 2C show conventional examples wherein negative lens elements are used in relay lens units for correcting longitudinal chromatic aberration. FIG. 2A shows the conventional example disclosed by the above-mentioned Japanese Patent Kokoku Publication No. Sho 49-5993 wherein negative lens elements which have a relatively high refractive index and a strong dispersing power are disposed in the middle between two rod-like cemented lens components so that spherical aberration is corrected by cemented surfaces that have a negative power and longitudinal chromatic aberration is corrected by the strong dispersing power of the negative lens elements. This conventional example requires at least four lens elements for composing a relay lens unit, or lens elements numbering as many as 12 for relaying an image three times. When the rod-like lens element of the relay lens unit is divided into thin plano-convex lens elements and a rod-like lens element having planar surfaces on both sides, as shown in FIG. 2B, taking surface eccentricies introduced during manufacturing into consideration, it will be required to use as many as eight lens elements for composing a relay lens unit, thereby stupendously enhancing the manufacturing cost for the relay lens system. Apart from this example, at least four lens elements are required for favorably correcting longitudinal chromatic aberration with a relay lens unit which includes a rod-like lens element.
As an example wherein longitudinal chromatic aberration is not corrected in a relay lens unit, there is known a lens system disclosed by U.S. Pat. No. 4,784,118. This conventional example has a composition shown in FIG. 2C wherein a relay lens unit is composed of lens elements in an extremely small number of 2. However, this conventional example wherein longitudinal chromatic aberration is not corrected in the relay lens unit can hardly correct longitudinal chromatic aberration in the relay lens system as a whole, thereby being incapable of providing good quality images.
As other conventional examples wherein longitudinal chromatic aberration is not corrected in relay lens units, there are known lens systems which use gradient index lens elements. A relay lens system disclosed by Japanese Patent Kokai Publication No. Sho 50-68349, for example, is configured so that longitudinal chromatic aberration produced by a gradient index lens element used in a relay lens unit is cancelled by another relay lens unit disposed on a side of an eyepiece system. However, a relay lens system comprising a gradient index lens element can hardly be manufactured so as to have a small outside diameter and such a relay lens system is unusable with non-flexible endoscopes which have small diameters, or is applicable only within a limited range. For configuring a relay lens system which is usable commonly with a variety of non-flexible endoscopes, it is therefore necessary to use optical elements which are obtained by polishing or shaping homogenous glass materials.
Further, the above-mentioned relay lens unit which is disposed on the side of the eyepiece system cannot be accommodated in the optical system holding tube used in the insert section and makes no contribution at all to extension of an image relaying distance in the insert section. It is therefore desirable that all lens elements used for composing a relay lens system have outside diameters which can be accommodated in the optical system holding tube.